1. Field of the Invention
The present invention relates to an inkjet printer capable of ejecting ink onto a recording medium to perform a printing operation.
2. Description of the Related Art
An inkjet printer well known in the art includes a sheet supply mechanism for supplying sheet to a prescribed printing area opposed to an inkjet head and performs a printing operation on the sheet at this printing area by ejecting ink through a plurality of nozzles onto the sheet. Of these inkjet printers, both serial type and line type inkjet printers are well known in the art. Line type inkjet printers include an inkjet head whose longitudinal dimension corresponds to the maximum width of sheet that can be printed (hereinafter referred to as the “maximum printable width”). The inkjet head prints on the sheet as the sheet is fed, in a direction orthogonal to the longitudinal direction of the head. However, when sheet having a narrower width than the maximum printable width of the inkjet head is printed, the sheet is printed with only specific nozzles corresponding to the printing area of the narrow sheet among all nozzles of the inkjet head, so that these nozzles will eject ink an extraordinarily high number of times. In other words, a specific region of actuator units that eject ink through the nozzles will be used very frequently at this local position. Hence, this specific region of actuator units will degrade more quickly than the rest, causing a decline in ink ejection accuracy from nozzles at this printing area and resulting in an overall decline in printing quality.
Therefore, inkjet printers capable of preventing a concentrated use of specific nozzles have been proposed. For example, an inkjet printer disclosed in Japanese unexamined patent application publication No. 5-162303 is configured to offset nozzles in the widthwise direction for each sheet supplied to a prescribed printing area when contour lines extending in a sheet supply direction are printed, as in CAD drawings (see FIG. 3). This type of inkjet printer can somewhat alleviate the high increase in usage frequency of nozzles printing contour lines.
The inkjet printer disclosed in Japanese unexamined patent application publication No. 5-162303 offsets the printing of nozzles in the widthwise direction of the sheet. However, since the position in which the sheet is supplied from the sheet supplying mechanism does not change, the position of images printed on the sheet is different for each sheet. Further, there is a limit on how far the nozzles can be offset in the widthwise direction in order to keep the printed images within the sheet. Accordingly, the usage of nozzles in a specific area of the printing head increases dramatically in frequency when printing is made within a relatively wide range of sheet having a narrower width than the maximum printable width of the inkjet head. Thus, the local ejection accuracy of ink in this area is reduced. Hence, later printing on wider sheet will give rise to a local area with inferior printing quality. Further, when narrow sheet is printed continuously, components constituting the sheet conveying mechanism, such as sheet conveying rollers and belts, will be used much more frequently in areas used for supplying the narrow sheet, thereby necessitating the early replacement of these parts.